The present invention generally relates to a signal converter for converting an analog signal into a pulse signal. Further, the present invention is concerned with a control system using the present signal converter.
A signal converter for converting an analog signal into a pulse signal is used in, for example, a detection circuit which detects a number of revolutions of a rotating member. A sensor provided in the detection circuit generates a sine wave detection signal having a frequency corresponding to a revolution of a rotating member. A signal converter converts the sine wave detection signal into a pulse signal having the same frequency as the sine wave detection signal. A revolution detector receives the pulse signal and measures the number of revolutions of the rotating member on the basis of the pulse width of the pulse signal.
Referring to FIG. 1A, there is illustrated a conventional signal converter which converts a sine wave detection signal indicative of the number of revolutions of a rotating member into a pulse signal having the same frequency as the sine wave detection signal. The signal converter shown in FIG. 1A is formed of a comparator 1. A sine wave detection signal S1 generated and output by a sensor (not shown) is applied to the comparator 1 via an input terminal Tin. The comparator 1 compares the sine wave detection signal with a reference voltage Vs, and generates a pulse signal S2 shown in FIG. 2. A revolution detector (not shown) is connected to the comparator 1 and detects a revolution of the rotating member on the basis of the pulse with of the pulse signal S2.
If the rotating member has an eccentricity, a low-frequency voltage component arising therefrom is superimposed on the pulse signal so that, as shown in FIG. 3, the sine wave detection signal S1 is changed to a pulse signal S3. When the pulse signal S3 having the low-frequency voltage component is input to the comparator 1, the comparator 1 outputs a pulse signal S4 having a pulse width which does not correspond to a variation in the frequency of the sine wave detection signal S2. Such a pulse width causes an error regarding the detected number of revolutions of the rotating member which is not negligible.
Conventionally, as shown in FIG. 1B, a filter circuit 4 having a fixed cutoff frequency is interposed between the input terminal Tin and the comparator 1. However, if the frequency of a signal to be detected changes in a wide frequency range, the filter circuit 4 does not operate effectively.
As shown in FIG. 4A, even if the cutout frequency, labeled CF, of the filter circuit 4 is set between a signal S to be detected and an unnecessary (noise) signal N having a low frequency due to the presence of an eccentricity for example, there is a possibility that the frequencies of the signal S and the noise signal N will change greatly and thus become higher than the cutoff frequency CF, as shown in FIG. 4B. In this case, the filter circuit 4 does not operate effectively. On the other hand, if the frequencies of the signal S and the noise signal N change greatly and thus become lower than the cutoff frequency CF, the signal S is attenuated together with the noise signal N.